We claim:
1. An inverse fourth power photometric calibrator comprising first means for emitting light flux as a quasi-point source from a first effective emission point; second mans, defining a first quasi-point detecting area spaced from said first emission point by a distance d, for emitting light flux as a quasi-point source from a second effective emission point and such that the total light flux emitted is proportional to the light flux received from said first light emitting means over said first detecting area; and means, defining a second quasi-point detecting area spaced from said second emission point by said distance d, for detecting the light flux received from said second light emitting means over said second detecting area.
2. The calibrator of claim 1 wherein said second light emitting means comprises photodetector means having a collecting aperture defining said first detecting area, a variable light flux output light source, and means for proportionally varying the output of said variable output light source in dependence on the output of said photodetector means.
3. The calibrator of claim 1 wherein said second light emitting means comprises means defining said first detecting area for collecting said light flux received from said first light emitting means, and means for mixing/diffusing the light flux collected by said collecting means and for emitting mixed light flux as a quasi-point source.
4. The calibrator of claim 3 wherein said mixing and emitting means comprises a plurality of incoherently-arranged optical fibers bundled together such that the respective ends thereof collectively constitute at a first end said flux collecting means and at a second end a quasi-point source from which said mixed flux is emitted.
5. The calibrator of claim 3 wherein said flux collecting means comprises means defining a collecting aperture, and said mixing and emitting means comprises a condenser lens.
6. The calibrator of claim 3 wherein said flux collecting means comprises means defining a collecting aperture, and said mixing and emitting means comprises at least one ground glass element and means defining an emitting aperture.
7. The calibrator of claim 4 wherein said first and second ends of said bundled fibers lie in a first plane, and said second detecting area and said first emission point lie in a second plane, and said second light emitting means is displaceable with respect to said second plane along an axis parallel to an optical axis between said first and second light emitting means.
8. The calibrator of claim 7 wherein the optical paths between said first and second emitting means and between said second light emitting means and said second detecting surface are optically isolated from each other and from the environment.
9. The calibrator of claim 1 wherein said first emission point and said second detecting area are disposed in a first plane, and said second emission point and said first detecting area are disposed in a second plane so as to form a folded optical path between said first emission point and said second detecting area.
10. The calibrator of claim 1 wherein said first and second planes are displaceable with respect to each other along an axis parallel to said folded optical path, thereby allowing the degree of attenuation of the light flux detected by said detecting means to be varied.